Coating process with alkali metal phosphate and added fluoride salt



Patented Jan. 5, 1954 COATING PROCESS WITH ALKALI METAL PHOSPHATE ANDADDED FLUORIDE SALT Paul Leslie Amundsen and Walter A. Osip, De-

troit, Mich., assignors, by mesne assignments, to Parker Rust ProofCompany, Detroit, Mich a corporation of Michigan No Drawing. ApplicationJune 17, 1949, Serial No. 99,884

4 Claims. 1

This invention relates to the formation of thin, uniform surfacecoatings for metals, such as iron, steel, and zinc. These coatings areprimarily suitable to increase the life of subsequent applied siccativecoatings and also are useful in facilitating drawing operations on iron,zinc and steels, such as in drawing dies.

The successful processes for such coatings, which have been adopted. byindustry, deal with the use of acid phosphate solutions of zinc,manganese and iron, used as is, or with suitable accelerators. Such aprocess is described in U. S. Patents Nos. 1,911,726 and 1,888,189.These solutions are all quite acid, having pHs ranging from 2 to 2.5.This is nece sary because these phosphates are soluble only in a fairlvacid medium. The disadvantages of these solutions are their corrosiveaction on equipment, dissolution of excessive amounts of iron andconsequent heavy sludge formation, due to the formation of insolubleiron phosphates.

The phosphate coatings formed in these processes can be divided into twodistinct groups. The heavy ones, ranging from 300 men-1000 mg. or overper souare foot, and the lighter ones ranging from 150-300 mg. The heavycoatings found adaption in the rustproofing of small parts, such asbolts. and nuts, and larger articles that were not painted. Thesecoatings were sealed with oils or waxes. It was found that these heavycoatings were unsatisfactory paint bases. For this reason, lighter oneswere developed. Bv proper selection of accelerators the coating weightscould be controlled to some extent, giving thinner films, which provedgood paint bases for sheet metal. The heavier coatings absorbed too muchpaint, dulling the film, and were le s flexible on bending, causing thepaint to flake off. However, all these solutions, containing largeamounts of zinc and iron, could not be controlled to give much thinnercoatings than approximately 150 mg. per square foot.

The phosphates discussed in the art above, are so-called coating metalphosphates. This means phosphates, the cations of which are metals whichenter into the coating proper, such metals as zinc, manganese and iron,thus forming insoluble. more basic phosphates. The alkali metalphosphates are in a difi'erent solubility group, in that they aresoluble, no matter what the acidity of the solution may be.

Attempts have been made to eliminate the disadvanta es outlined above byusing these more soluble and cheaper phosphates, such as Q the alkalimetal phosphates. These show possibilities, but as such, do not coat asuniformly and as well as necessary.

We have found how to utilize the property of these cheaper, moresoluble, less acid phosphates, together with a fluoride. These havequite special properties. The fluorides form acid and normal salts, andhave the tendency to form insoluble complex compounds.

It is therefore, an object of the invention to provide an inexpensivemethod of preparing metal for painting using solutions having a pH of3.0 or over to minimize the dissolution of iron and to eliminate anycorrosive action on equipment.

Still another object is to .provide a method of the above type by whichan adherent coating is provided on iron, zinc or steel surfaces in arelatively short time, the produced coating being effective to improvethe drawing properties of the metals thus facilitating its cold drawingin manufacturing operations. increasing the life of the dra ing dies andeouipment and permitting the making of deeper dra s in a sin leoperation than is normally possible without the coatings.

Another obiect is to provide a method and coating solution of the abovetvpe adapted to eliminate the sludge caused by zinc or iron in solutionand also adapted to form thinner coatings which are better all aroundpaint bases.

Another obiect of the present invention is to provide a novel processand coating solution for producing an adherent coating on metalsurfaces, such as iron and zinc. by chemical means, the process beingreadilv adapted for use on an economically feasible basis in commercialscale operations.

Other obiects of the invention will appear in the following descriptionand appended claims. Before explaining the present invention in detail,it is to be understood that the invention is capable of otherembodiments and of being practiced and carried out in various ways. Itis also to be understood that the phraseology or terminology employedherein is for the purpose of description and not of limitation.

In using the mono alkali metal or mono ammonium phosphates inconjunction with alkali or ammonium fluorides, or bifluorides, coatingsolutions can be made up for iron, steel or zinc, which are all bufferedand form thin uniform films in a very short time. The solutions areparticularly suitable for spray applications in a conventional type ofspraying machine, for such parts as automobile bodies, refrigeratorpanels 3 and the like. A one minute spray at 140150 F. on properlycleaned articles is very adequate.

Immersion application normally requires a longer time, usually from 3 tominutes at 150-'l80 F. for best results on steel and one minute or lesson zinc.

A certain ratio between the phosphate and fluoride is necessary for bestresults. For instance, a 1:1 ratio by weight is recommended. Thephosphate should not be allowed to drop below of the total coatingchemical in solution and may go as high as 95%, atwhich point theremaining fluoride still exerts its beneficial activating action.

A proper balance is maintained in using a mix of fluoride and acidfluoride. A proper balance is important because too much normal fluoridetends to decrease the acidity, whereas too much acid fluoride gives toomuch acid for proper coat ing action.

The above coatings may be rinsed with dilute chromic or phosphoric acidif desired.

The following examples are given to illustrate the invention:

ExampleI The following ingredients in the parts by weight indicated werefirst mixed and then dissolved in water to give a suitable coatingsolution in accordance with the present invention.

.46% mono sodium phosphate 27% sodium fluoride 27% sodium bifluoride Incoating metals with the above ingredients, 17 pounds of the above drymix were used per 100 gallons of processing solution at 160 F. Thesolution was tested for acid strength by titrating a 10 ml. sample withN/lO of sodium hydroxide, using phenolphthalein indicator. The end pointwas a permanent pink and was reached with about 15 m1. of sodiumhydroxide. This is called a 15 point solution. The pH as measured with astandard glass electrode was 4.25.

Example II The following ingredients in the parts by weight indicatedwere first mixed and then dissolved in water to form another suitablecoating solution.

20% mono ammonium phosphate 40% sodium fluoride 40% sodium bifluoride Inpreparing a coating solution using the above ingredients, 20 pounds ofthe above dry mix were used for each 100 gallons of solution. The pH ofthe solution was found to be 4.3, measured with a standard glasselectrode.

Example III The following ingredients in the. parts by weight indicatedwere first mixed and then dissolved to form a suitable coating solutionin accordance with the present invention.

90% mono potassium phosphate 5% sodium fluoride 5% ammonium bifluorideIn employing the above ingredients, 1'7 pounds of the dry mix were usedper 100 gallons of solution. Suitable coatings were obtained on steeland zinc articles which were immersed or subjected to a spray of thiscoating solution for three and one minute periods, respectively, at 150F. The pH of this solution was found to be approximately 4.4. By varyingthe relative amounts of the above constituents, the pH of the solutionmay be varied up to 5.5, using more normal fluoride and less bifluoride.

Example IV The following ingredients in the parts by weight indicatedwere first mixed and then dissolved in water to form another suitablecoating solution.

45% mono ammonium phosphate 35% sodium fluoride 20% sodium bifluoride Inemploying the above ingredients in a coating solution, 20 pounds of theabove dry mix were employed per 100 gallons of final solution. The pH ofthis solution was found to be 5.3. Care should be exercised to maintaina pH for the final processing bath of between 3.0 and 5.8. Too low a pHgives too much pickling, whereas, too high a pH, naturally, stops allthe action. This may be accomplished by changing the relativeproportions of the fluoride and bifluoride.

In the prepartion of the coating solution, any combination of alkali oracid compounds of the activators may be employed, so balanced as to givethe solution a pH between 3.0 and 5.8. For example, a suitable amount ofsodium fluoride may be combined with a suitable portion of sodiumbifluoride, or hydrofluoric acid, to provide the desired balance.

The exact chemical reactions of the above solutions are not known, butundoubtedly fluoride ions activate the metal surface in thesecomparatively high pH coating solutions. The coatings thus formed arecombinations of phosphates and oxides, with traces of insoluble halogen,and are improvements over the coatings obtained with straightphosphates, and permits operation of the process at pI-Is much higherthan have been possible heretofore.

The coatings obtained by the method of the present invention are thinand uniform, weighing only, 25-100 mg. per square foot. They areidealbases for subsequent organic finishes, especially when a final rinseapplication of chromic acid and di-chromates, or phosphoric acid isused. These rinse acids are used in very low concene tration, .05% to.10% solution by weight and are allowed to dry onto the coating.

The coatings of the present invention have also been found particularlysuitable to .facilitate drawing operations, by increasing thelife of thedraw dies and equipment and also by permitting deeper draws in a singleoperation.

The term alkali metal in the subsequent claims includes the alkalimetals and also ammonium.

Having thus described our invention, we claim:

1. In a method for coating metal, surfaces, such as iron, steel, zincand alloys thereof, the novel step of subjecting the metal to the actionof an, aqueous coating solution which essentially consists of an alkalimetal phosphate, and an alkali metal fluoride until a coating is formedon the surface of the metal, the concentration of the phosphate in thesolution being between 15 and percent by weight of the total chemical insolution, said solution having a pH between 3.0 and 5.8.

2. In a method for coating metal surfaces, such as iron, steel, zinc andalloys thereof, the novel step of subjecting the metal to the action ofan aqueous-coating solution which essentially consists of an alkalimetal phosphate, and an activator comprising alkali fluorides and alkaliacid fluorides, until a suitable coating is formed thereon, theconcentration of the alkali fluoride and alkali acid fluoride beingbalanced to maintain the pH of the solution between 3.0 and 5.8, theconcentration of the phosphates in the solution being between 15 and 95percent by weight of the total chemical in solution.

3. A method for coating metal surfaces such as iron, steel, zinc andalloys thereof comprising the steps of subjecting the metal to theaction of an aqueous coating solution which essentially consists of analkali metal phosphate, having therein an alkali fluoride until acoating is formed on the surface of the metal, removing the metal fromthe coating solution and thereafter rinsing the coated surface with adilute acid of the group consisting of chromic and phosphoric acids, theconcentration of the phosphates in the solution being between 15 and 95percent by weight of the total chemical in solution, said solutionhaving a pH between 3.0 and 5.8.

4. In a method for coating metal surfaces such as iron, steel, zinc andalloys thereof, the novel References Cited in the file of this patentUNITED STATES PATENTS Number 7 Name Date 1,022,274= Strecker Apr. 2,1912 1,989,312 Gerber Jan. 29, 1935 2,105,015 Singer Jan. 11, 19382,203,670 Buzzard June 11, 1940 2,312,855 Thompson Mar. 2, 19432,316,220 Brown et a1 Apr. 13, 1943 2,477,841 Ward Aug. 2, 19492,479,423 Snyder Aug. 16, 19%9 2,494,910 Spruance, Jr. Jan. 17, 19502,500,673 Gibson et al. Mar. 14, 1950

2. IN A METHOD FOR COATING METAL SURFACES, SUCH AS IRON, STEEL, ZINC ANDALLOYS THEREOF, THE NOVEL STEP OF SUBJECTING THE METAL TO THE ACTION OFAN AQUEOUS COATING SOLUTION WHICH ESSENTIALLY CONSISTS OF AN ALKALIMETAL PHOSPHATE, AND AN ACTIVATOR COMPRISING ALKALI FLUORIDES AND ALKALIACID FLUORIDES, UNTIL A SUITABLE COATING IS FORMED THEREON, THECONCENTRATION OF THE ALKALI FLUORIDE AND ALKALI ACID FLUORIDE BEINGBALANCED TO MAINTAIN THE PH OF THE SOLUTION BETWEEN 3.0 AND 5.8, THECONCENTRATION OF THE PHOSPHATES IN THE SOLUTION BEING BETWEEN 15 AND 95PERCENT BY WEIGHT OF THE TOTAL CHEMICAL IN SOLUTION.